In the Gulf of Guinea, he told CNN's "The Lead," slow-moving vessels servicing oil platforms are more vulnerable to attacks than cargo ships traveling off the coast of Somalia, another area that's drawn attention for maritime piracy.

The Gulf of Guinea produces some 5.4 million barrels of oil a day, according to Chatham House. And about 30% of U.S. oil imports flow through the region, according to International Crisis Group.

The oil-rich area off the coast of West Africa has increasingly drawn international attention as a piracy hotspot, with 40 pirate attacks reported in the first nine months of 2013, the International Maritime Bureau reported.

It also has been the site of the only ship crew kidnappings worldwide this year, with 132 crew members taken hostage.

Seven ships have been hijacked, the organization said.

Piracy in the Gulf of Guinea accounted for 30% of the 1,434 reported piracy attacks in African waters between 2003 and 2011 and the pace of attacks has risen since then, London-based think tank Chatham House reported in March.

Chatham House reported 62 pirate attacks in the gulf in 2012, up from 39 in 2010.

The think tank says it's partially because Western navies have cracked down on piracy off the coast of Somalia, on the other side of the continent. But it's unclear whether any troops will intervene after this week's attack in the Gulf of Guinea.

The Nigerian Navy has directed its operational commands and bases to search and rescue the crew members and the vessel, spokesman Commodore Kabir Aliy said.

U.S. Marines are in the region aboard a Dutch ship off West Africa. Military forces from the United States, United Kingdom, Spain, the Netherlands and five African nations recently held exercises in the region that were designed to strengthen maritime security, according to the U.S. Navy.

"We are seeking additional information about the incident so that we may contribute to safely resolving the situation," State Department spokeswoman Marie Harf told reporters Thursday. "Obviously our concern at this point is for the safe return of the two U.S. citizens."